U.S. intelligence suggests that the Russian plane crash in Egypt's Sinai was most likely caused by a bomb planted on the jet by Islamic State or one of its affiliates, CNN reported Wednesday, citing a U.S. official.
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CNN said the official familiar with the matter said the U.S. intelligence community had not yet reached a formal conclusion on the cause of the crash.
"There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane," CNN quoted the official as saying.
The Russian-operated Airbus A321M crashed on Saturday shortly after taking off from the Egyptian resort Sharm al-Sheikh on its way to St. Petersburg, killing all 224 people on board.
Earlier Wednesday, Britain said that the passenger plane might have been brought down by an explosive device.
"While the investigation is still ongoing we cannot say categorically why the Russian jet crashed," Prime Minister David Cameron's office said in a statement. "But as more information has come to light we have become concerned that the plane may well have been brought down by an explosive device."
As a precautionary measure, the British government has decided that flights due to leave Sharm for Britain on Wednesday evening will be delayed to allow time for a team of U.K. aviation experts, currently travelling to Sharm, to make an assessment of the security arrangements in place at the airport.
Britain's assessment of the crash came during a visit to London by Egyptian President Abdel-Fattah al-Sissi.
Egypt's Civil Aviation Ministry said Wednesday that investigators have extracted and validated the contents of the flight data recorder, one of two so-called black boxes recovered from the airplane.
The ministry said the second black box which contains the cockpit voice recorder (CVR) was partially damaged and much work was required to extract data from it.
An Egyptian source close to the investigation of the black boxes told Reuters that the cause of the crash is looking more like an explosion but it is not clear whether it was linked to fuel or engine trouble or a bomb.
"It is believed to be an explosion but what kind is not clear. There is an examination of the sand at the crash site to try and determine if it was a bomb," the source said.
"There are forensic investigations underway at the crash site. That will help determine the cause, to see if traces of explosives are found."
A Russian aviation source told Reuters that investigators are looking into the possibility that an object stowed on board caused the disaster.
"There are two versions now under consideration: something stowed inside (the plane) and a technical fault. But the airplane could not just break apart in the air - there should be some action. A rocket is unlikely as there are no signs of that," the source said.
ISIS takes credit
Meanwhile, Islamic State's Egyptian affiliate dismissed in an audio message doubts that it had downed the Russian passenger plane over Egypt's Sinai Peninsula, killing all aboard, and said it would tell the world how it did so in its own time.
Sinai Province, an Egyptian group loyal to Islamic State, said in a statement the same day that it had brought down the airliner "in response to Russian air strikes that killed hundreds of Muslims on Syrian land".
The claim was dismissed by Russian and Egyptian officials. Security experts and investigators have said the plane is unlikely to have been struck from the outside and Sinai-based militants are not believed to possess the technology to shoot down a jet from a cruising altitude above 30,000 feet.
Russian officials have, however, said the plane probably broke up in the air, leaving open the prospect of some kind of explosion on board.
Asked to comment on those remarks and local press reports that the black box voice recorders had picked up unusual sounds before the crash, Civil Aviation Minister Hossam Kemal said the facts had yet to be established.
"This is all speculation. There is nothing definitive until the investigation commission completes its probe," he said.
In an audio message posted on a Twitter account used by the group, Islamic State's Egyptian affiliate insisted it was behind the crash. The claim could not immediately be authenticated.
"We, with God's grace, are the ones who brought it down, and we are not obliged to disclose the mechanism of its demise," the speaker said.
Russia, an ally of Syrian President Bashar Assad, launched air raids against opposition groups in Syria including Islamic State on Sept. 30. The hardline group has called for war against both Russia and the United States in response to their air strikes in Syria.
Islamic State backers in Iraq issued a video on Tuesday congratulating their Egyptian colleagues and warning Russian President Vladimir Putin that more was to come.
Sinai-based militants have killed hundreds of Egyptian soldiers and police in recent years and have also attacked Western targets. Egypt has carried out air strikes on them.
Islamic State websites have in the past claimed responsibility for actions that have not been conclusively attributed to them. Officials say there is no evidence to suggest so far that a bomb brought down the plane.